The government has pledged that 10% of the UK's energy will come from renewable energy sources by 2010 - a promise backed up by a £100m boost announced by Prime Minister Tony Blair on Tuesday.

Mark Johnston, Climate Solutions Campaigner with Friends of the Earth (FoE), says while environmentalists welcome Mr Blair's announcement, they would like to see the 10% target doubled.

But at present less than 3% of Britain's electricity comes from alternative sources and to Ian Fells, Professor of Energy Conversion at Newcastle University, achieving the target by 2010 would be "difficult".

BBC News Online asked both men to weigh up the benefits and problems of different sources of renewable energy.

Hydroelectric power

Almost all the UK's electricity generated from renewable sources comes from the two hydroelectric dams in Scotland.

Benefits: Very powerful. A tried and tested form of energy production which has already proved successful.

Problems: Requires a dam to be built which environmentalists say risks disturbing river or estuarine ecosystems.

The structures are also large and expensive to build.

"We have run out of space for large hydro," says Ian Fells.

Wind

Wind power has created a lot of interest in recent years and is one of the world's fastest growing energy technologies.

There are about 60 wind farms around the UK and the first off-shore farm was opened last year off Blyth, Northumberland.

The use of wind power is rising rapidly

However, just 0.25 of the Britain's energy needs are currently met through wind power.

Benefits: The UK is one of the windiest countries in Europe so it makes sense to harness the energy.

According to Mark Johnston, the UK has the potential to provide three times its current energy requirements with wind power.

We could rely more heavily on this source in the winter when the weather is windier.

It is also cheap to harness.

Problems: Each wind turbine is large - about 70m across - and some people object to the idea of them dotting the landscape.

They generate a relatively low amount of power and Professor Fells reckons 1,500 turbines would have to be built by 2010 for 2.5% of our energy to come from the wind.

The wind does not blow all the time so we would need to use a battery technology to store the energy, which is expensive to do.

Tidal and wave energy

Waves can be used to turn a generator or turbine - as on Islay where the UK's first, and only, commercial wave power station was opened last November.

Tides can be used to fill a hydroelectric dam.

Benefits: As an island nation, Britain has a huge coastline which it could use for these forms of power.

The UK's coastline could be harnessed

A US company is considering building a plant to harness tidal power off the Welsh coast, using the aggregate from slag heaps to build a "hollow island" which would have its own regular tides.

"This would benefit the environment in two ways: by clearing up the Welsh countryside and by providing clean reusable energy," says Mark Johnston.

Problems: The Islay power station only generates 500kw - a relatively low rate of power.

Ian Fells says 10,000 such stations would be needed around the Scottish coast to create as much power as the two nuclear power stations in Scotland.

There are few sites with a great enough difference between low and high tides to make harnessing tidal power possible.

Solar

Energy from the sun can be harnessed in solar cells - also called photo-voltaic cells. These can be small enough to meet specific energy needs such as heating a house's water or grouped together in large banks.

Solar energy is expensive at present

Benefits: While solar energy is expensive to harness at present, it is rapidly coming down in price. Mark Johnston says it should be as cheap as wind within a decade.

We could rely on this source more in the summer although at least one company has developed a cell which can be used in low light and possibly even moonlight.

Problems: It is expensive to harness so it would need a high level of subsidy to make it viable.

As with wind, this is an intermittent source of energy which might need a battery technology to make it reliable.

Biomass or crop fuel

Fast growing plants such as willow or elephant grass can be harvested and turned into woodchips which can then burnt in power stations.

Benefits: A "smart source of energy" says Mark Johnston - it is cheap, easy and quick to develop.

"We also hope that biomass could be a way to rescue the rural economy," he says.

In a combined heat and power plant (CHP), as well as turning a turbine in the same way as fossil fuels, the heat created, which is normally discarded as a worthless byproduct, could be used to provide steam for industrial processes or hot water and heating for rooms at the power station. This would greatly increase energy efficiency.

Problems: Huge amounts of crops would need to be grown in order to make the use of biomass worthwhile.

"To provide half the electricity which comes from Dungeness nuclear power station in Kent, you would have to cover the whole of Kent with trees," says Ian Fells.